The invention is situated in the field of Quasi Synchronous Code Division Multiple Access combined with Time Division Duplexing.
CDMA (Code Division Multiple Access) is one of the leading technologies in today""s and future wireless and wireline communications systems.
Also known as Direct Sequence Spread Spectrum, CDMA is the best known representative of the class of spread spectrum modulation schemes. A CDMA waveform is generated by spreading (EXOR-ing) the data stream with a PN-code, resulting in a higher bandwidth, usually at lower power spectral density. Different users are multiplexed by using orthogonal or quasi-orthogonal codes.
CDMA is used for a variety of reasons. CDMA has its origin in the military, where use of PN-codes (Pseudo Noise) was exploited for the sake of its Low Probability of Intercept (LPI) or its Low Probability of Detection (LPD). Since the late eighties, civil applications started to be developed, and have now reached a high level of maturity and market penetration. Advantages of spread spectrum include the inherent interference rejection capabilities, the efficient way for multiplexing multiple services, the higher capacity, a more efficient use of the spectrum and the lower terminal costs.
In satellite communications, CDMA is being exploited for low rate, medium rate as well as broadband type of communications. Low rate applications include, e.g., SMS (Short Messaging Services), E-mail over satellite, remote meter reading, voice and data services (fax), and positioning and geolocation applications.
When used in the VHF and UHF bands, one talks about the so-called xe2x80x98Little LEOxe2x80x99 (Low Earth Orbit) applications. CDMA is used for its capability of coping with high interference levels in these bands, and because it allows to multiplex a high amount of users with limited protocol overhead. In order to provide global coverage, they are usually store-and-forward satellite systems with sophisticated OBP (On-Board Processing) capabilities.
Data and fax services using CDMA are found in the L and S bands (in the case of the so-called xe2x80x98big LEOxe2x80x99 constellations) and Ku-bands (when using transponders of GEO satellites). Different systems can share the same part of the spectrum. Cost-effective terminals are possible by exploiting a high degree of on-chip integration.
The Ku-band frequencies are mostly used by geostationary satellites, for applications such as DBS (Direct Broadcasting by Satellite). With the enormous growth of the demand for medium rate data services (mostly for the transport of Internet data), transponders in Ku-band are increasingly used also for implementing these data services (multiples of 64 kbps net user data rate).
CDMA-based solutions have an important cost advantage over the traditional PSK-based VSAT solutions. Ground station development is facilitated using the CDMA DataSat Development System. Other application examples in the Ku-band include the combination of terrestrial low cost networks (based on DECT) with a S-CDMA (Synchronous CDMA) satellite for multiplexing the different telephone channels. In more and more cases, CDMA is being used as an overlay to existing satellite services. This is a very efficient use of spread spectrum in view of the limited spectrum being available. One such example is D-SNG (Digital Satellite News Gathering), where the CDMA-based coordination channels are put on top of the transponder QPSK DVB (Digital Video Broadcasting) signals.
Besides the ongoing developments using existing GEO capacity, LEO-based satellite systems are under development to provide true broadband access to individual users, using CDMA technology. Several Mbit/s can be offered to the individual user. In some cases, these spread spectrum high-rate applications share the spectrum with other, non-CDMA services, by realizing links with very low power spectral density.
Probably the best known satellite application using CDMA technology is navigation. Both GPS (Global Positioning System) and GLONASS (GLObal Navigation Satellite System, the Russian counterpart) use Direct Sequence Spread Spectrum waveforms for obtaining accurate pseudo-range measurements, which are the raw data to calculate a position fix. GPS has been complemented with the EGNOS (European Geostationary Navigation Overlay System) and the American WAAS (Wide Area Augmentation System), to increase the performance.
Proposed European GNSS-2 (Global Navigation Satellite System) will provide higher accuracy and increased data rate using more sophisticated waveforms. Besides the vast consumer market for standard receivers, there""s also a professional market for RTK (Real-Time Kinematic) receivers. These applications exploit the increased accuracy associated with tracking (or pseudo-tracking) of the P-Code(Precision Code) of GPS or GLONASS satellites. Alternative navigation systems are being developed, based on combined pseudo-range and Doppler measurements.
In wireless terrestrial communications, one can make a distinction between applications in licensed and in unlicensed bands.
In licensed bands, the best known system under development is the UMTS (Universal Mobile Telecommunications System), based on W-CDMA (Wideband CDMA) technology. Worldwide, this development fits in the IMT-2000 (International Mobile Telephone System) initiative from the ITU, which aims at realizing a true worldwide applicable 3G standard. The benefits of CDMA exploited here are the ability to merge different types of services (voice, data, video) over the same band, using orthogonal PN sequences of different lengths, leading to the best capacity(bits/Hz). This so-called 3G (3rd Generation) cellular networks will be commercially exploited from 2001 on, as an extension to the worldwide deployed GSM networks.
The provision of the license-free ISM(Industrial, Scientific and Medical) bands has boosted a lot of other terrestrial wireless applications. Well-known ISM bands are the 902-928 MHz band in the US, and the 2.4-2.4835 GHz band worldwide. CDMA is used here for its ability to share the same spectrum with other applications. Short range and Long range communications are being realized in these shared bands, efficiently rejecting the MAI (Multiple Access Interference). Data rates of several hundreds of kilobits per second are possible in this way.
Furthermore, applications such as from CATV modems and powerline modems also benefit CDMA, as once again the unwanted unpredictable interference (e.g., ingress noise cancelled) is efficiently through the processing gain of the spread spectrum modulation scheme.
U.S. Pat. No. 5,872,810 and European Patent Application EP-A-0767544 describe a flexible hardware platform on which any PN code family can be downloaded on on-chip RAM; the PN code properties and their influence on the performance of a CDMA link can be analyzed with this platform and these documents further describe a CDMA transceiver integrated circuit on which any PN codes can be stored on on-chip RAM.
De Gaudenzi et al. describe in U.S. Pat. No. 5,327,467 a CDMA-based system primarily of interest for mobile communications.
In U.S. Pat. No. 5,327,455, they describe a QPSK/CDMA modulation scheme, using preferentially phased Gold codes for spreading the data streams;
In R. De Gaudenzi, C. Elia and R. Viola, xe2x80x9cBandlimited quasi-synchronous CDMA: A novel access technique for mobile and personal communication systems,xe2x80x9d IEEE Selected Areas in Communications, vol. 10, no. 2, pp. 328-348, February 1992, CDMA-base satellite communications system exploiting Quasi-Synchronous CDMA in order to obtain a high efficiency together with interference rejection capabilities are described.
One aspect of the present invention is a method of constructing orthogonal codes of length N for use in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing, the method comprising a) determining the balanced vectors of length N, being all possible cross-correlation vectors resulting from zero cross-correlation of codes of length N; b) providing an arbitrary code of length N; c) performing bitwise XOR-ing with all the balanced vectors determined in a), to produce a set of codes with which the arbitrary code is orthogonal; d) performing bitwise XOR-ing of the balanced vectors determined in a); e) adding the code to a set of orthogonal codes if the result of d) is balanced; f) performing a)-e) until the set of orthogonal codes is complete; and g) applying the set of orthogonal codes to a plurality of data streams in the network so as to provide spread spectrum data streams.
Another aspect of the present invention is a method of deriving alternative sets of orthogonal codes for use in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing, the method comprising a)representing a given orthogonal code set as a matrix in N*N matrix format with N being an even number for binary codes, and being an integer for non-binary codes; and b)performing at least one of the following on the N*N matrix of a):
Exchanging columns in the matrix;
Exchanging rows in the matrix;
Inverting columns in the matrix;
Inverting rows in the matrix;
so as to obtain an alternative set of orthogonal codes.
A further aspect of the present invention is a method of deriving a subset out of N codes of length N, with N being an even number for binary codes and being an integer for non-binary codes, for use in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing, for which all mutual cross-correlation functions are zero over more than one subsequent point, starting with the first point of the cross-correlation function, the method comprising a)selecting an orthogonal code set; b) deriving alternative sets of orthogonal codes for use in the network so as to group zeros in the cross-correlation function as the first points of the cross-correlation function; c) selecting the solutions which have a well-peaked response by investigating auto-correlation functions during the search process; and d) applying the solutions selected in c) to a plurality of data streams in the network so as to provide spread spectrum data streams.
Another aspect of the present invention is a method of deriving M orthogonal codes of length N+M for use in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing, the method comprising concatenating a set of M orthogonal codes of length N and a set of M orthogonal codes of length M, respectively.
Another aspect of the present invention is a method of constructing binary codes for use in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing from a combination of two different sets of codes, the method comprising providing a first code of one of the sets as a generator; providing a second code of one of the sets as a seed; replacing a bit of the first code by the corresponding bit of the second code if the corresponding bit is a zero, or replacing the bit by the bit-inverse of the corresponding bit of the second code if the corresponding bit is a one; and applying the resulting code to a plurality of data streams in the network so as to provide spread spectrum data streams.
Another aspect of the present invention is a method of fast acquisition of auto-correlation peaks for use in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing, using codes with equally-spaced peaks, the method comprising searching for a auto-correlation peak; testing for the validity of the choice; if the choice is not correct, testing the next auto-correlation peak; and if the choice is correct, applying the chosen codes to a plurality of data streams in the network so as to provide spread spectrum data streams.
Another aspect of the present invention is a method of obtaining a higher aggregate network capacity through reduction of the cross-correlations between concurrent transmissions in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing, the method comprising extending a direct spread symbol with a partial duplication of the direct spread symbol, resulting in even cross-correlation.
Another aspect of the present invention is time division duplex communication system using synchronous code division multiple access (S-CDMA) communication in the forward link between one master station and multiple slave stations, and quasi synchronous code division multiple access (QS-CDMA) in the return link between the multiple slave stations, the system comprising a master station transmit cycle configured to synchronize the slave stations to a beacon signal transmitted by the master station, acknowledge each of the codes by the one master station from a slave station transmit cycle received from any successfully demodulated slave stations, and transmit direct spread data to any earlier already acknowledged slave stations; the slave station transmit cycle configured to transmit, by each slave station having obtained the synchronization, a code selected from a list to the master station, or transmit data, direct spread by the code from slave stations already having received the acknowledgement from the master station transmit cycle; and a repeat loop to return the system to the master station transmit cycle.
Another aspect of the present invention is a method of constructing non-binary digital orthogonal code sequences of length N, with N any natural number, for use in a network utilizing quasi-synchronous code division multiple access combined with time division duplexing, the method comprising providing a unity base in an N-dimensional vector space, V, having operations comprising addition of vectors (+) and scalar product of vectors (.); applying consecutive base transformations to the base; representing the resulting base vectors by their coordinates in the N-dimensional vector space to obtain a set of orthogonal codes; and applying the set of orthogonal codes to a plurality of data streams in the network so as to provide spread spectrum data streams. The base transformations can comprise selecting two base vectors out of the N base vectors; determining the plane defined by the two base vectors; and rotating the two base vectors over the same angle in the plane.